Intertidal Zone:
Life Between the Tides

Epaulette Shark

Named for the conspicuous dark patch on each ‘shoulder’, the diminutive
and prettily-patterned Epaulette Shark (Hemiscyllium ocellatum) is as
appealing as it is unusual. This species features an elongate, eel-like body
with small, flexible posterior fins and highly mobile paired fins.

The paired pectoral and pelvic fins of Epaulette Sharks are broadly
rounded and paddle-like. The internal skeleton of these paired fins and
their attachment to the body has been modified, granting them a dramatically
increased range of motion — they can actually be rotated almost 360 degrees.
Epaulette Sharks combine side-to-side wriggling with coordinated pectoral
and pelvic fin movements, enabling them clamber over complex rocky and coral
substrates.

The wriggling gait of the Epaulette Shark has been studied as a model of
the probable limb movements used by the first tetrapods (four-footed
vertebrates) to clamber from the sea onto land. This research provides
evidence supporting the evolutionary theory that the paired limb movements
necessary for terrestrial locomotion pre-date the first amphibians. No
matter how important it may be from a theoretical standpoint, the ‘walking’
motion employed by Epaulette Sharks is endearingly clumsy, resembling that
of an elongate salamander with ping-pong paddles for feet.

The elongated body and ambulatory talents of Epaulette Sharks enable them
to crawl through the cramped, meandering passageways within coral reefs. The
shallow pools exposed by each low tide often offer rich feeding on the small
animals trapped within, but are so extremely oxygen poor that few aquatic
predators can exploit them. Recent research has revealed that, under
extremely low-oxygen conditions, Epaulette Sharks maintain blood flow to the
brain by selectively dilating the blood vessels leading to it. This simple
mechanism enables Epaulette Sharks to ‘walk’ for hours at a time among
exposed tidepools and to make do with what little dissolved oxygen these
warm little puddles provide. In addition, Epaulette Sharks’ nerves are
adapted to function under low-oxygen conditions for prolonged periods. Their
ability to compensate for hypoxic conditions enables Epaulette Sharks to
feed in exposed reef flats and tide-pools during low tide.

The diet and feeding behavior of Epaulette Sharks was recently studied on
the shallow reef flats off Heron Island, Australia. The researchers found
that over 90% of Epaulette Sharks’ diet consists of just two prey types:
crustaceans and segmented worms. They also found that immature Epaulette
Sharks concentrate on different prey types than mature ones. Juvenile
Epaulette Sharks feed predominantly on polychaete worms, while adults feed
predominantly on crabs. Round-the-clock observations indicate that Epaulette
Sharks may feed at any time during day or night, but do so mostly at dawn
and dusk. From this and other observations on Epaulette Shark feeding
behavior, the researchers concluded that this species is opportunistic
(feeding on whatever it happens to find), relying mostly on scent and
electroreception to locate prey.

The shallow-water feeding behavior of Epaulette Sharks is absolutely
delightful to watch. Clambering awkwardly over the substrate in its adorably
salamander-like way, the Epaulette Shark uses is short snout to snuffle
through sandy patches rather like a determined pig searching for truffles.
Epaulette Sharks often adopt a peculiar stance while foraging over soft
substrates: the body is elevated on the paddle-like pectoral and pelvic fins
while the back is arched and the ‘neck’ bent down — resembling a selachian
form of “push ups”. Stopping occasionally, the shark pushes its head into
the sand and excavates buried prey through a powerful suction affected
through coordinated action of its puckered mouth and thick gill musculature.

Sometimes, a foraging Epaulette Shark will flip over bottom debris with
its snout, exposing hidden worms and crustaceans which are then quickly
snarfed up. One naturalist reported seeing an Epaulette Shark wedge the
anterior part of its body into a coral crevice, turn itself over and hoover
up a small shrimp from the roof of the crevice. Apparently, the elongate
Epaulette Shark occupies a niche more like that of moray eels than does any
other shark.

The Epaulette Shark is also unusual in that it often chews its food
before swallowing, sometimes for as long as five or even ten minutes. If the
food item is relatively large, it often protrudes from the shark’s mouth,
migrating from side-to-side like the stogie of an archetypal big-time
Hollywood producer.

Clambering into exposed tidepools creates exceptional feeding
opportunities for Epaulette Sharks. Most notably, it adds to the menu
‘exotic’ fishes, trapped by the receding tide in confined spaces from which
there is no escape. Any small crabs, worms, fishes, or other creatures
trapped inside a tidal puddle need merely be cornered against a wall of the
pool and they can be captured easily.

Unfortunately for the Epaulette Shark, whenever there is easy feeding in
nature, invariably others will come to exploit it, too. Every once in a
while, a moray or snake eel wriggles its way into a tidepool and begins to
feed on the bounty of small creatures trapped there. But what happens when
one of these eels and an eel-like Epaulette Shark wind up hunting in the
confined quarters of the same pool? Usually, one or the other competitor
will simply move to another tidepool. But on rare occasions, a fight breaks
out, the shark and the eel biting and wrapping around one another as they
writhe like angry snakes. More often than not, the little Epaulette Shark
concedes victory and clambers away as quickly as its paddle-like fins will
carry it.

The shallow-water and intertidal meanderings of Epaulette Sharks also
bring them into contact with people. Due to their small size and endearingly
clumsy locomotion, Epaulette Sharks are pathetically easy for beachgoers to
catch. Captured individuals squirm vigorously, usually eliciting laughter
and squeals of delight from human on-lookers. Although they cannot inflict
serious damage on their tormentors, Epaulette Sharks are often injured in
the ordeal. As is far too often the case with our interactions with
wildlife, these encounters often provide human amusement to the detriment of
the sharks.